1. Field of the Invention
The present invention relates to the fabrication of semiconductor electronic integrated circuits, and, more particularly, to the fabrication of III-IV compound semiconductors containing heterojunction bipolar transistors.
2. Description of the Related Art
Most high speed microwave heterojunction bipolar transistors (HBTs) require self-aligned emitter-base contacts to minimize series base resistance and to reduce total base-collector capacitance. The present methods can be broadly divided into two groups. In the first group are those methods employing chemical etching and in the second group are those employing dry etching techniques.
When using chemical etching processes, the emitter contact cannot be used as a mask for etching through the emitter layer to the base layer due to galvanic effects. Therefore, the methods of the first group always use re-aligned emitter contacts, which places a limit on the minimum emitter size that can be used, typically 2 microns.
The dry etching techniques of the second group use reactive ion etching (RIE) or ion milling to remove the unwanted portions of the emitter layer. Either angle evaporation or sidewall insulators are then used to ensure controlled emitter-base separation. The angle evaporation method is limited to approximately 1.5 micron emitter geometries, whereas the ion mill/sidewall method introduces too much damage to the emitter-base interface due to high energy bombardment of the surface.
Thus there is a need for a method of fabricating emitter-base contacts of a HBT in a self-aligned process that does not limit emitter size and can accommodate different thicknesses of emitters.